The aging of polymers is of considerable importance to, among others, industrial and electrical power plant operators in that the unanticipated failure of such polymers may have significant adverse effects on human safety, plant operation and maintenance costs and downtime. Polymers are used in key components related to the safe and reliable operation of industrial and power plants. Specifically, polymers are found in, but not limited to; cables, pumps, valves and seals.
Electrical and optical cables, such as power, control, instrumentation and data transmission have traditionally been considered long-lived components which merit little in the way of preventive maintenance or condition monitoring due to their generally high level of reliability and simplicity of construction. Like all other components, however, such cables age as a result of operational and environmental stressors. The typical modes of degradation due to cable aging are embrittlement leading to cracks, loss of dielectric strength, and increased leakage current. The main stressors causing age-related degradation are thermal aging resulting from elevated temperatures and ionising radiation. Other degradation stressors of cables include mechanical stresses, humidity, hydrocarbon fluids, and ozone.
Aging effects may be spatially generalized (i.e., affecting most or all portions of a given cable equally, such as for a cable located completely within a single room of uniform temperature), or localized (i.e., affecting only very limited portions of a cable, such as in the case of a cable routed near a highly localized heat source). The severity of these aging effects depends on several factors including the severity of the stressor, the materials of construction and design of the cable, and the ambient environment surrounding the cable. Detailed discussions of electrical cable aging may be found in a number of publications including Kim, J-S., “Evaluation of Cable Aging in Degradation Based Plant Operating Condition” (2005) J. Nucl. Sci. Technol. 42(8) 745-753 and SAND96-0344 “Aging Management Guideline for Commercial Nuclear Power Plants—Electrical Cable and Terminations” prepared by Sandia National Laboratories/U.S. Department of Energy, September 1996. Discussions regarding optical cable aging may be found, inter alia, in Electric Power Research Institute (EPRI) publications and telecommunications industry literature. The following description will be limited to electrical cable, although it can be appreciated that the principles of aging and analysis described herein may also be largely applicable to optical cabling.
A typical instrumentation and control (I&C) cable consists of multi-conductor assemblies insulated with fire-retardant material with an overall shield and an outer jacket. In addition, the cables used in plants such as nuclear reactor stations may contain tape wraps that enhance electrical, mechanical, or fire protection properties.
Insulation and jacket materials used for I&C cables are polymers that contain additives and fillers to improve aging resistance, electrical, mechanical and fire retardant properties. The most widely used jacket and insulation materials in older CANDU® plants are polyvinyl chloride (PVC). In newer plants the materials are chlorosulphonated polyethylene (CSPE), also know as Hypalon™ for the jackets and cross-linked polyethylene/polyolefin (XLPE/XLPO), and ethylene-propylene based elastomers (EPR, EPDM) for the insulation.
The level of degradation of the insulation and jacket materials attributed to aging depends upon the polymer compound used (presence of adequate additives, etc.), the pre-service (storage) and service environmental conditions (temperature, radiation, mechanical stress, humidity), and the elapsed service life (time factor). The main chemical aging mechanisms of polymers result from scission, cross-linking, and oxidation reactions at the molecular level. The scission of alkoxyl or peroxide radicals usually leads to the scission of one macromolecular chain into two new chains. Cross-linking refers to the formation of covalent links between adjacent macromolecules and the formation of a dense network of chains. Oxidation reactions, which start from the formation of free radicals (because of the initial break of a covalent link under the effect of temperature and/or radiation), can lead either to chain scission or cross-linking. The organic materials usually undergo physical changes such as hardening and loss of flexibility as a result of exposure to heat and radiation. Another type of physical aging mechanism due to thermal aging is the evaporation and possible migration of plasticizers in PVC materials.
The level of degradation of a material can be assessed by tracking the changes of material properties. Some standard techniques used include: visual and tactile inspections, tensile tests, indentation tests, differential scanning calorimetry, Fourier Transform Infrared Reflectance (FTIR) Spectroscopy, measurement of swelling ratio, mass loss, plasticizer content, dielectric measurements or change in density.
One of the most commonly used laboratory techniques to assess degradation is tensile testing, which consists of comparing the percentages of elongation at break (EAB) or the tensile strength for unaged and aged samples. EAB is a proven degradation indicator and an accepted parameter for the estimation of the residual lifetime of a cable. End-of-life criteria based on this parameter are well established. An ultimate EAB of 50% is usually used as an end point criterion [International Atomic Energy Agency, 2000, “Assessment and Management of Ageing of Major Nuclear Power Plant Components Important to Safety: In-Containment Instrumentation and Control Cables”, Volume 1, IAEA-TECDOC-1188, December]. The main disadvantage is the large sample size required and the destructive aspect of the technique.
The number of techniques available for on-site monitoring is limited because of the strong requirement from station personnel to use non-destructive and non-intrusive techniques. Another difficulty is that some of the instruments typically used in the laboratory environment are not easily portable to site.
Over the past few years, various panels of international experts were formed to review existing data and the state of advancement of current condition monitoring techniques [IAEA-TECDOC-1188, 2000 (above) and Nuclear Energy Agency, Committee on the Safety of Nuclear Installations, 2004, “Research Efforts Related to Wire Systems Aging in NEA Member Countries”, Report NEA/CSNI/R, (2004)12, August 11]. These panels provided guidelines and recommendations with respect to the orientation of Research and Development (R&D) programs to address cable aging issues. The recommendations for future research and development efforts to address this issue were as follows [Report NEA/CSNI/R, 2004 (above)]:                Continue the development of new, effective, in-situ condition monitoring techniques for installed wire systems that can be used to determine the current condition of a wire system and predict its useful life. In this regard, advanced electrical, optical, ultrasonic and aerospace technologies should be evaluated and developed for nuclear plant applications; and        Correlate mechanical wire system properties to electrical properties to better understand the significance of reaching the limits of mechanical properties for aged insulating materials.        
Some of the physical techniques used to analyse cable polymer aging, such as the measurement of the tensile strength or elongation-at-break of the insulation material are inherently destructive and require a specimen of the aged cable for testing. However, there are some non-destructive physical techniques, including the measurement of compressive modulus, torsional modulus, or rigidity under bending, that do demonstrate a correlation between the aging of the cable and the measured parameter (especially for low-voltage cable), and can be practical to apply during operational conditions. For example, the measurement of compressive modulus by way of instruments such as the Indenter Polymer Aging Monitor can be useful for measurement of cable polymer aging. See, for example, EPRI TR-104075, “Evaluation of Cable Polymer Aging Through Indenter Testing of In-Plant and Laboratory Aged Specimens,” prepared by the Electric Power Research Institute, January, 1996 for a discussion of the correlation between outer jacket and conductor physical measurements.
The portable indenters currently used are generally limited to the sole measurement of material stiffness or hardness. However, for some polymer-based materials, the stiffness/hardness remains unchanged with increasing irradiation level, even though basic material properties such as the elongation at break clearly indicate a continuous degradation resulting from this stressor. Likewise, when polymeric components are subjected to thermal aging, the stiffness sometimes increases initially but quickly reaches a saturation value, even though it is known that further degradation continues to occur. Therefore, the indenters currently available are not ideally suited for the monitoring of cable aging.
In an indenter made by Electric Power Research Institute (EPRI) the limit of indentation depth is controlled based on the value of the force measured. Therefore the indentation depth varies between an unaged and an aged elastomer. This prevents the study of recovery of the elastomer for a fixed reference indentation depth. The EPRI indenter can be used to monitor a portion of the force signal after the maximum force is reached and the force starts to relax and decay. The probe can be held in position during the relatively short relaxation period that is being analysed. However, the force relaxation features do not change significantly with increased aging of the material.
With the EPRI indenter, once the relaxation information is acquired, the probe is slowly driven back to original position and no further investigation takes place. Because of the nature of the drive system, the probe cannot be retracted instantly or quickly from a given reference position. Therefore it is not possible to create conditions that permit assessment of recovery of deformation following the force relaxation phase when using the EPRI indenter.
In addition, current portable indenters do not offer the flexibility of changing the type of excitation signals, nor programming a variety of sequences of events for the indenter probe. This is detrimental to the systematic identification of optimal input parameters, set-ups, and output parameters in terms of their sensitivity to polymer degradation.
Based on the foregoing, there remains a need for a method and device for monitoring and estimating the aging of polymer cable, which method and device is portable, non-destructive and permits optimization and measurement of characteristics other than merely polymer stiffness.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.